Callable horses using horns!
The idea is simple, using one of the following crafting recipes, you can craft a Horse horn, (yes, the name of the item needs work, but just listen). Once you have a horse horn, you can right click on a tamed horse, and link it to the horn, which will cause the horse's name to appear written underneath the name of the horse horn, roundabout where you would find enchantment names on an enchanted sword. Right click the horn again, and it summons that horse to your side.
Works anywhere, anytime, aside from a slight cooldown to prevent people spamming it.